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RECIFE, Brazil | An Air France memo to its pilots Friday about the crash of Flight 447 said the airline is replacing instruments that help measure airspeed on all its medium- and long-haul Airbus jets.

Investigators have focused on incorrect speed readings as one potential factor in the crash.

With Brazil and France disagreeing about whether pieces of the jet have even been found in the Atlantic Ocean, investigators are using the last messages sent by the plane to determine the cause of the crash and try to avoid future disasters.

Air France declined to comment on the memo obtained by Associated Press, saying it was for pilots only.

Airbus said the matter was part of the investigation into the crash that killed 228 people flying from Rio de Janeiro to Paris Sunday. The Bureau of Investigation and Analysis, which is leading the French probe of the crash, said it would address all questions at a Saturday press conference.

The memo sent Friday said Air France has been replacing instruments known as Pitot tubes and will finish in “coming weeks.” It does not say when the replacement process started.

Pitot tubes are L-shaped metal tubes — about eight inches long on their longer side — that protrude from the wing or fuselage of a plane. The pressure of the air entering the tube allows sensors to measure the speed and angle of the flight, along with less vital information like outside air temperature.

They are heated to prevent icing.

The plane’s “black boxes” may be miles below the surface of the oceanand investigators are looking for clues in the messages sent from the plane’s computers just before it disappeared. One theory: the outside probes that feed speed sensors may have iced over, giving incorrect information to the plane’s computers. The autopilot may have then directed the plane to fly too fast or too slow when it met turbulence from towering thunderstorms.

Brazilian officials have insisted for three days that military pilots have spotted wreckage from Flight 447 scattered across the ocean’s surface. Air Force Brig. Gen. Ramon Cardoso again expressed confidence Friday that at least some of the objects — an airplane seat, a slick of kerosene and other items — are from the plane that vanished Sunday with 228 people on board.

“This is the material that we’ve seen that really was part of the plane,” Gen. Cardoso said.

But ships guided by planes in the search area have been hampered by extremely poor visibility, and have recovered no wreckage. The only piece retrieved so far, a cargo pallet, turned out to be sea garbage.

French Transportation Minister Dominique Bussereau said his own country’s searchers have found no signs of the Airbus A330. A French Defense Ministry official also questioned the Brazilian claims, saying French teams “cannot precisely confirm the zone where the plane went down.”